Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 4-121260, a brake force control apparatus is known which increases a power ratio of a brake booster when a brake pedal is depressed at a speed exceeding a predetermined speed. A driver of a vehicle operates a brake pedal at a high speed when a rapid increase in a brake force is desired. Hereinafter, such a brake operation is referred to as an emergency brake operation.
The above-mentioned conventional brake force control apparatus determines that the emergency brake operation is being performed when the brake pedal is operated at a speed which is greater than a predetermined speed. In this case, a relatively high brake pressure as compared to that generated during a normal brake operation is generated by pressurizing brake fluid stored in a reservoir and supplying the pressurized brake fluid to wheel cylinders. Hereinafter, this brake control is referred to as a brake assist control (abbreviated as a BA control).
According to the above-mentioned conventional brake force control apparatus, when the emergency brake operation is performed, an advantageous situation can be established for a rapid increase in the brake force by generating a relatively large power ratio as compared to that generated during a normal brake operation. Thus, when the driver requests a rapid increase in a brake force, it is possible to correctly satisfy the request.
In order to generate a relatively high brake pressure as compared to that generated during the normal brake operation when the emergency brake operation is performed by the driver, it is necessary to provide the brake force control apparatus with a means for pressurizing the brake fluid. A pump can be used as such a means. However, when a pump is used as a high pressure source, the increase in the brake force may be delayed due to a delay in the response of the pump, if the pump is started to be operated after a condition for starting the BA control is established. In this case, it is impossible to generate a brake force which appropriately satisfies the request by the driver.